Gather materials: 4 Sand and 4 Gravel
Gather dye (optional for colored concrete): any dye item
Craft concrete powder: place 4 Sand and 4 Gravel in a crafting table in a pattern that results in 8 Concrete Powder
Add dye to concrete powder: place Concrete Powder and the dye in a crafting table to get colored concrete powder
Place concrete powder blocks: place the concrete powder in the world
Harden it: expose the placed concrete powder to water to turn it into concrete
Repeat for more blocks as needed